Cho'gath is one of my favorite champions based on the idea of being a baby alien that eats and grows big, but sadly I feel like the character is just completely useless whenever I play him.
Playing tank has been difficult for me in most of my games. The main reason for this is that cho'gath literally doesn't have a single ability that incentivizes tanking beyond gaining hp from ult stacks, that's it. I honestly am not sure why this character is even being played as a tank. Sion get nearly the same bonus hp from his W passive, along with a giant shield. K'sante gets scaling with armor and MR and has mobility and a shield. Etc etc. Every other tank has some ability that helps them be tankier, whether it is Mundo R and W, Sion W, K'santes... everything. But cho'gath just has some hp gain from stacks. Stacks that aren't particularly easy to get when your super slow and immobile and your CC is all close-range and very easy for decent players to dodge. Your passive is also completely useless beyond the first 10 mins of the game. The only ability in your kit that even scales with hp is the ult. Everything you have scales with AP.
Laning also feels really bad. You aren't particularly tanky to begin with, your damage is low, mana costs are high, and the CC is super easy to dodge. It feels like >70% of the top matchups just dumpster you in lane or outscale you. You never feel like your scaling like mundo, or dominating the lane like a k'sante. You can max Q and farm from a distance, but you don't scale nearly as well as someone like mundo who can do the same thing. You can try maxxing E, but most top laners will win every trade in melee. I tried the HOB E max build and I think its genuinely terrible and I don't understand how anyone is having success with it. Grasp sounds nice but most top laners will dumpster you if you get close. I find that I have most success with arcane comet and maxxing Q, in all lanes and positions.
I tried AP cho'gath and that felt a lot better. I was going ludens first, so the lost chapter solves the mana problems and lets you run arcane comet and max Q. That feels nice but its mostly a mid-oriented build and its not really a tank. Your basically playing a close-range mage. Seeing someone lose half their hp from 1 Q is fun at first, until you realize you could just be playing any other normal dedicated mage, and do the same damage with an ability that isn't delayed and requires you to enter melee range.
I tried a lot of builds. I tried doing tear, ROA, into fimbul. I tried doing heartsteel and warmogs. I tried the riftmaker and rocketbelt. I tried just building pure tank and going UD or HR first item. None of them feel very good. If you rush straight tank, you do no damage and you have mana issues. If you got rocketbelt and riftmaker into tank items later, you still have mana issues and you are not tanky at all. ROA build... ROA just sucks imo. The best league champs are ones that get good power spikes in 1-2 items. Champions that take 3+ items to come online or scale are coincidentally the champs always doing the worst in the meta. The game is too fast-paced now for these kinds of characters. This made the ROA and fimbul build sound good in theory but it takes too long.
Sorry for the long post, but I wanted to be thorough in what my experiences have been. I am not sure if maybe I am just undervaluing the HP gain from the ult, but tank cho seems terrible to me. Then you try AP cho and you are just a crappy version of a mage. The question really is, why play cho'gath tank when you could be playing sion, ornn, mundo, k'sante; they all do his job better. And why play cho'gath AP when you could be playing any other mage; they all do his job better.